1. Field of the Invention
This invention relates generally to patterned-media magnetic recording disks, wherein each data bit is stored in a magnetically isolated data island on the disk, and more particularly to a method for making a master disk to be used for nanoimprinting the patterned-media disks.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In patterned media, the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric data tracks. To produce the required magnetic isolation of the patterned data islands, the magnetic moment of spaces between the islands must be destroyed or substantially reduced to render these spaces essentially nonmagnetic. In one type of patterned media, the data islands are elevated regions or pillars that extend above “trenches” and magnetic material covers both the pillars and the trenches, with the magnetic material in the trenches being rendered nonmagnetic, typically by “poisoning” with a material like silicon (Si). In another type of patterned media, the magnetic material is deposited first on a flat disk substrate. The magnetic data islands are then formed by milling, etching or ion-bombarding of the area surrounding the data islands. Patterned-media disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, or perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer.
One proposed method for fabricating patterned-media disks is by nanoimprinting with a template or disk, sometimes also called a “stamper”, that has a topographic surface pattern. In this method the magnetic recording disk substrate with a polymer film on its surface is pressed against the template. The polymer film receives the reverse image of the template pattern and then becomes a mask for subsequent etching of the disk substrate to form the pillars on the disk. In one type of patterned media, the magnetic layer and other layers needed for the magnetic recording disk are then deposited onto the etched disk substrate and the tops of the pillars to form the patterned-media disk. In another type of patterned media, the magnetic layers and other layers needed for the magnetic recording disk are first deposited on the flat disk substrate. The polymer film used with nanoimprinting is then pressed on top of these layers. The polymer film receives the reverse image of the template pattern and then becomes a mask for subsequent milling, etching or ion-bombarding the underlying layers. The template may be a master disk for directly imprinting the disks. However, the more likely approach is to fabricate a master disk with a pattern of pillars corresponding to the pattern of pillars desired for the disks and to use this master disk to fabricate replica templates. The replica templates will thus have a pattern of recesses or holes corresponding to the pattern of pillars on the master disk. The replica templates are then used to directly imprint the disks. Nanoimprinting of patterned media is described by Bandic et al., “Patterned magnetic media: impact of nanoscale patterning on hard disk drives”, Solid State Technology S7+ Suppl. S, SEP 2006; and by Terris et al., “TOPICAL REVIEW: Nanofabricated and self-assembled magnetic structures as data storage media”, J. Phys. D: Appl. Phys. 38 (2005) R199-R222.
In patterned media, the bit-aspect-ratio (BAR) of the pattern or array of discrete data islands arranged in concentric tracks is the ratio of track spacing or pitch in the radial or cross-track direction to the island spacing or pitch in the circumferential or along-the-track direction. This is the same as the ratio of linear island density in bits per inch (BPI) in the along-the-track direction to the track density in tracks per inch (TPI) in the cross-track direction. The BAR is also equal to the ratio of the radial dimension of the bit cell to the circumferential dimension of the bit cell, where the data island is located within the bit cell. The bit cell includes not only the magnetic data island but also one-half of the nonmagnetic space between the data island and its immediately adjacent data islands. The data islands have a ratio of radial length to circumferential width, referred to as the island aspect ratio (IAR), that can be close to or greater than the BAR.
The making of the master template or disk is a difficult and challenging process. The use of electron beam (e-beam) lithography using a Gaussian beam rotary-stage e-beam writer is viewed as a possible method to make a master disk capable of nanoimprinting patterned-media disks with a BAR of about 1 with a track pitch (island-to-island spacing in the radial or cross-track direction) of about 35 nm, and an island pitch (island-to-island spacing in the circumferential or along-the-track direction) of about 35 nm. If the data islands have a radial length and circumferential width each of about 20 nm for an IAR of 1, then these dimensions generally limit the areal bit density of patterned-media disks to about 500 Gbit/in2. To achieve patterned-media disks with both an ultra-high areal bit density (greater than 1 Terabits/in2), a track pitch and an island pitch of about 20 nm will be required. However, a master disk capable of nanoimprinting patterned-media disks with these small dimensions over an area equal to the data area of a disk is not achievable with the resolution of e-beam lithography.
Directed self-assembly of block copolymers has also been proposed for making the master disk and is believed capable of achieving areal bit densities of greater than 1 Terabit/in2. U.S. Pat. No. 7,976,715 B2, assigned to the same assignee as this application, describes the use of directed or guided self-assembly of block copolymers to form a pattern of generally radial lines on a master disk substrate, followed by conventional lithography to form a pattern of concentric rings over the radial lines. After removal of resist and one of the block copolymer components, the substrate has a pattern of pillars of the other block copolymer component, which are then used as an etch mask to etch the substrate.
What is needed is a master disk and a method for making it that can result in patterned-media magnetic recording disks with the required high areal bit density.